The Spector laboratory has recently developed a protocol for purification of interchromatin granule clusters (IGCs), also called "nuclear speckles", from mouse liver. Because we have already-begun to characterize the constituent proteins, I am in an excellent position to study the importance of this nuclear domain for supplying pre-mRNA synthesis and processing factors to active genes in mammalian cells. My proposal will address two fundamental questions about IGCs. First, I will determine if IGC integrity is necessary for splicing factors to be recruited to an active gene and catalyze intron excision in-vivo. Second, I will assess the nature of IGC organization to determine if IGCs occur at non-random sites in living cells, and determine if novel proteins play a structural role in IGCs. This study will advance our understanding of how gene expression is spatially organized in mammalian nuclei.